The present invention relates to an AC motor control device driven by n (n.gtoreq.2) inverters. More particularly, the present invention relates to a load-commutated inverter (to be referred to as a thyristor motor hereinafter) for operating a synchronous motor having one or more sets of three-phase windings, wherein a deviation in electrical angles of 60 degrees/n exists in the outputs from the n inverters.
A thyristor motor comprises a combination of a static frequency converter and a synchronous motor. A thyristor motor has advantages common to those of an AC motor, i.e., easy maintenance and high reliability. At the same time, a thyristor motor allows variable operation within a wide range of speed as in the case of a DC motor by changing the frequency of the converter. In a thyristor motor of this type, in order to prevent commutation failure of the converter, the advanced control angle of the thyristor is automatically adjusted. A control device for the thyristor motor automatically adjusted in this manner is disclosed, e.g., in Japanese Patent Publication No. 55-27556 (July 21, 1980). In this conventional control device, the advanced control angle of the thyristor is controlled such that the commutation margin angle is kept constant.
However, in a thyristor motor wherein a synchronous motor having one or more sets of three-phase windings is driven by n (n.gtoreq.2) inverters, a problem is encountered if the above-mentioned control method is adopted. This problem will be described with reference to a case wherein n=2 (60 degrees/n=30 degrees).
FIG. 1A shows changes in an advanced control angle .beta. and a commutation overlapping angle u in a conventional thyristor motor for a synchronous motor having two sets of three-phase windings. Each of angles .beta. and u is shown as a function of current Ia which represents the value of currents flowing in the windings of the respective phases. These changes are such that the commutation margin angle .gamma. (=.beta.-u) is kept constant. Referring to FIG. 1A, 30 degrees along the axis of ordinate represents an electrical angle corresponding to the phase deviation between the two sets of three-phase windings.
FIG. 1B shows changes in the effective commutation margin angle .gamma.* as a function of current Ia. Note that the effective commutation margin angle .gamma.* is an electrical angle corresponding to an actual reverse biasing voltage for commutating thyristors in each inverter. Referring to FIG. 1B, Ia1 along the axis of abscissa represents the value of current when advanced control angle .beta. is 30 degrees.
FIGS. 1A and 1B teach that when advanced control angle .beta. exceeds 30 degrees during .gamma.-constant control, effective margin angle .gamma.* is reduced and thyristor commutation is rendered unreliable. In other words, the conventional thyristor motor cannot guarantee stable operation in the control range of .beta.&gt;30 degrees.
Effective commutation margin angle .gamma.* is reduced in the control region exceeding 30 degrees for the following reasons:
FIG. 2 shows an example of a voltage waveform applied to the thyristor. Referring to FIG. 2, when one thyristor in the inverter is considered, it is turned on at time t1 and is turned off after time t2. In FIG. 2, region "a" indicates a reverse biasing portion for forcibly turning off the thyristor. When the reverse biasing portion "a" is enlarged, it is as shown in FIG. 3C (.beta.&lt;30 degrees) or in FIG. 4C (.beta..gtoreq.30 degrees).
FIGS. 3A and 3B show changes in the motor current which are related to the reverse biasing voltage in FIG. 3C. FIGS. 4A and 4B show changes in the motor current which are related to the reverse biasing voltage waveform shown in FIG. 4C. FIGS. 3A and 4A show the relationship between a current IU1 flowing to the phase U1 of the first set of windings (U1, V1 and W1) and a current IV1 flowing to the V1 phase thereof. FIGS. 3B and 4B show the relationship between a current IU2 flowing to the U2 phase of the second set of windings (U2, V2 and W2) and a current IV2 flowing to the V2 phase thereof. The second set of three-phase windings has a phase deviation of 30 degrees with respect to the first set of three-phase windings.
As may be seen in FIGS. 3A to 3C, when the values of currents IU1, IV1, IU2 and IV2 are small (corresponding to the case of Ia&lt;Ia1 in FIG. 1B), a constant commutation margin angle .gamma. is obtained within a range of .beta.&lt;30 degrees. In this case, control is performed such that .gamma. (=.beta.-u)=.gamma.*=constant, and the thyristor is operated stably.
As shown in FIGS. 4A to 4C, when the values of currents IU1, IV1, IU2 and IV2 increase (corresponding to the case of Ia .beta. Ia1 in FIG. 1B), commutation overlapping angle u also increases and angle .beta. exceeds 30 degrees while performing .gamma.-constant control. Then, a voltage change (having the opposite polarity to that of the reverse biasing voltage of the thyristor) occurring from the motor upon commutation from current IU2 to IV2 overlaps or breaks into the region of commutation margin angle .gamma. (FIGS. 4B and 4C). For this reason, the electrical angle which corresponds to the actual reverse biasing voltage period used for commutation from the U1 phase to the V1 phase becomes .gamma.* (.gamma.=.beta.-u&gt;.gamma.*) which is smaller than constant .gamma..
In the .beta. control region exceeding 30 degrees, effective commutation margin angle .gamma.* becomes smaller than the constant, nominal commutation margin angle .gamma.. Accordingly, if the control target of commutation margin angle .gamma. is set to a value corresponding to the minimum time required for commutation of the thyristor, commutation failure occurs and stable thyristor motor operation cannot be performed in the control region of .beta.&gt;30 degrees. However, if the control target of angle .gamma. is set to be sufficiently larger than the above-mentioned minimum value so as to prevent such commutation failure, the reverse biasing voltage application period becomes excessively long for the region of .beta.&lt;30 degrees and the efficiency (or power factor) of the thyristor motor is lowered. The above is the first disadvantage of the conventional thyristor motor of .gamma.-constant control type.
When rotational speed N and/or the load of the motor abruptly changes, in the conventional thyristor motor, the control operation of advanced control angle cannot follow such an abrupt change. In this case, effective commutation margin angle .gamma.* may temporarily decrease during this period and commutation failure may occur. This is the second disadvantage of the conventional thyristor motor which does not incorporate any measure against abrupt change in rotational speed/load of the motor.
Note that the second disadvantage described above also occurs in a thyristor motor wherein a synchronous motor having one set of three-phase windings is driven by a single inverter. However, this disadvantage is more critical in a thyristor motor wherein a synchronous motor having one or more sets of three-phase windings is driven by n (n.gtoreq.2) inverters. A thyristor motor of n.gtoreq.2 is required irrespective of the first disadvantage because ripples in the generated torque obtained in the case of n.gtoreq.2 are lower than those obtained in the case of n=1.